Ultrasound stimulated acoustic emission (USAE) imaging was recently proposed by Fatemi and Green-leaf for detection of the mechanical frequency response of tissues. This technique exploits the interference of two overlapping focused ultrasound beams with slightly different frequencies. The interference induces a localized low frequency vibration deep in tissue at the ultrasound focus. The vibrating tissue acts as an acoustic source with a magnitude that depends on the mechanical response of the tissue. Our initial experiments with ex vivo tissues have demonstrated that this method can detect temperature elevations of tissue and the stiffness change associated with tissue coagulation. However, the USAE is sensitive to ambient noise and other experimental resonant conditions. To overcome these problems we plan to combine the ultrasound stimulation method with a separate ultrasound beam that will track the tissue motion in the focal zone. Our hypothesis is that this new ultrasound local harmonic motion (LHM) imaging eliminates the drawbacks associated with the USAE method while retaining its sensitivity to the mechanical properties of tissues. We will use the R21 phase to evaluate the performance of this new imaging method for thermal therapy monitoring. If this phase is successful, then our plan in the R33 phase is: First, to develop a theoretical simulation model and use it to investigate the parameters influencing system performance for diagnosis and therapy monitoring. Second, to construct a prototype device based on the simulation results. Third, to perform phantom measurements to optimize the performance of the device and to quantify its resolution and to compare the method with MRI thermometry. Fourth, to design and construct an applicator that can be used to coagulate prostate tissue transrectally. Finally, to test the applicator performance and feasibility of the proposed method by performing sonications of prostate surgery in dogs (in vivo). The potential benefits of this research are significant. This technique could make monitoring of minimally invasive thermal and ultrasound therapies much less expensive than the current use of magnetic resonance imaging. In addition, the phased array development will provide an effective method for testing LHM imaging in an electronically scanned 2 D field for diagnostic purposes. Therefore the potential clinical impact of the proposed research is significant.